Serotonin is present in a fine beaded plexus in the cerebellar cortex of several mammalian species. In the cat, serotoninergic afferents arise from neurons located within the lateral, paramedian and peri-olivary reticular nuclei (Kerr and Bishop, J. Comp. Neurol., 304 (1991) 502-515). In addition to serotoninergic afferents, these same nuclei also contain a separate population of neurons that give rise to mossy fibers to the cerebellar cortex. Physiological studies have shown that mossy fibers are excitatory to their target neurons. The intent of the present study was to determine the physiological effects of serotonin in the cat's cerebellum in an in vivo preparation and to identify the receptor(s) that mediate the observed responses. Iontophoretic application of serotonin (5HT) onto Purkinje cells reduces the spontaneous firing rate of all cells tested (n = 12). Serotonin also blocks the excitatory effects elicited by the application of aspartate in 17 of 19 units tested and of glutamate (n = 62) in all cases. In addition, 5HT potentiated the inhibitory action of GABA (n = 12). Iontophoretic application of the 5HT1A agonists, 8-OH-DPAT and ipsapirone, mimic the suppressive action of serotonin in a dose-dependent manner. This response, as well as the 5HT mediated suppression are blocked by the application of spiperone, a 5HT1A antagonist. Compounds selective for the 5HT1C,2 and 3 receptors are physiologically ineffective. The present data are in partial agreement with previous studies in the rat's cerebellar cortex.(ABSTRACT TRUNCATED AT 250 WORDS)